130x1g2 - CCSDS

TM SYNCHRONIZATION AND CHANNEL CODING—SUMMARY OF CONCEPT AND RATIONALE3 TM SYNCHRONIZATION AND CHANNEL CODING3.1 INTRODUCTIONChannel coding 1 is a signal processing technique by which data can be sent from a source toa destination through a noisy channel so that distinct messages are easily distinguishablefrom one another. This allows reconstruction of the data with improved reliability.In spacecraft, the data source is usually digital, with the data represented as a string of‘zeros’ and ‘ones’. A channel encoder (or simply ‘encoder’) is then a device that takes thisstring of binary data and produces a modulating waveform as output. If the channel code ischosen correctly for the particular channel in question, then a properly designed decoder willbe able to reconstruct the original binary data even if the waveforms have been corrupted bychannel noise. If the characteristics of the channel are well understood, and an appropriatecoding scheme is chosen, then channel coding provides higher overall data throughput at thesame overall quality (bit error rate) as uncoded transmission - but with less energy expendedper information bit. Equivalently, channel coding allows a lower overall bit error rate thanthe uncoded system using the same energy per information bit.There are other benefits that may be expected from coding. First, the resulting ‘clean’channel can benefit the transmission of compressed data. The purpose of data compressionschemes is to map a large amount of data into a smaller number of bits. Adaptivecompressors will continually send information to direct a ground decompressor how to treatthe data that follows. An error in these bits could result in improper handling of subsequentdata. Consequently, compressed data is generally far more sensitive to communication errorsthan uncompressed data. The combination of efficient low error rate channel coding andsophisticated adaptive data compression can result in significant improvement in overallperformance (reference [6]).Second, a low bit error rate is also required when adaptive (or self-identified) telemetry isused. Adaptive telemetry is much like adaptive data compression in that information on howvarious ground processors should treat the transmitted data is included as part of the data. Anerror in these instructions could cause improper handling of subsequent data and the possibleloss of much information.Third, low error probability telemetry may allow a certain amount of unattended missionoperations. This is principally because the operations systems will know that any anomaliesdetected in the downlink data are extremely likely to be real and not caused by channelerrors. Thus operators may not be required to try to distinguish erroneous data from genuinespacecraft anomalies.In a typical space channel, the principal signal degradations are due to the loss of signalenergy with distance, and to the thermal noise in the receiving system. The codes describedin reference [3] can usually provide good communication over this channel.1 This technique is called ‘channel’ coding because it is adapted to the statistical behavior of the channel and itapplies to the overall transmitted data stream, not to specific sources only.CCSDS 130.1-G-2 Page 3-1 November 2012